Ecological Risk Assessment Step 7

Sections of Step 7 Baseline ERA (BERA) Risk Characterization:

Risk Characterization

Risk Characterization is the seventh step of the Superfund Ecological Risk
Assessment process and
includes two major components: risk estimation and risk description. Risk
characterization should be well-balanced, clear, reasonable, consistent, easy
to follow and understand, with all assumptions, uncertainties, and professional
judgments clearly identified.

Risk Estimation

Data interpretation methods should be presented in the risk characterization
documentation. For example, if the triad approach (i.e., toxicity test,
benthic invertebrate survey, and sediment chemistry) was used to evaluate
contaminated sediments, the risk estimation section should describe how
the three types of studies are integrated to draw conclusions about risk.

In addition to developing point estimates of exposure concentrations,
it might be possible to develop a distribution of exposure levels based
on the potential variability in various exposure parameters. This is called
a dose-response curve, where likely responses (i.e., harmful effects)
can be predicted from specific levels of contamination. Probabilities
of exceeding a threshold for adverse effects might then be estimated.

Risk Description

Risk description: provides information important for interpreting
the risk results and identifies a level for harmful effects on the assessment
endpoints.

A key to risk description for Superfund sites is the documentation of
environmental contamination levels that bound the threshold for adverse
effects on the assessment endpoints. In other words, what are the NOAELs and LOAELs for specific animals and
plants that are affected at the site under investigation. The risk description
can also provide information to help the risk manager judge the likelihood
and ecological significance of the estimated risks. It is important to
document the contaminant concentrations in each environmental medium (soil,
sediment, water) that bound the threshold for estimated harmful ecological
effects, because there is uncertainty inherent in the data and models
used. The lower bound of the threshold would be based on consistent conservative
assumptions and NOAEL toxicity values. The upper bound would be based
on observed impacts or predictions that ecological impacts could occur.
The upper bound would be developed using consistent assumptions, site-specific
data, LOAEL toxicity values, or an impact evaluation.

In addition to identifying one or more thresholds for effects, the risk
assessment team might develop estimates of the probability that exposure
levels would exceed the ecotoxicity thresholds given the distribution
of values likely for various exposure parameters.

The risk assessor should also put the estimates in context with a description
of the extent, magnitude, and potential ecological significance of those
estimates. Additional ecological risk descriptors are as follows:

The location and areal extent of existing contamination above a threshold
for harmful effects;

The degree to which the threshold for contamination is exceeded or
is likely to be exceeded in the future; particularly if exposure-response
functions are available;

How long it would take for the contaminants to disappear from the
environment, either from decay, breakdown, or physically moving out
of the site;

The potential for natural recovery once the sources of contamination
are removed.

Uncertainty Analysis

There are several sources of uncertainties associated with ecological risk
estimates. One is the initial selection of Contaminants of Potential Ecological
Concern (COPECs) based on the sampling data and available toxicity information.
Other sources of uncertainty include estimates of toxicity to ecological
receptors at the site based on limited data from the laboratory, other ecosystem,
or over a limited period of time from the site; the exposure assessment
as a result of uncertainty in chemical monitoring data and models; and estimates
of risk when simultaneous exposures to multiple COPECs occur. These sources
fall into three basic categories of uncertainties:

Conceptual model uncertainties;

Natural variation and parameter errors;

Modeling error.

Conceptual model uncertainties (CSM):

The initial description of the ecological problems at a site, likely exposure
pathways, COPECs, and exposed ecological components required a degree of
professional judgment and assumptions, which results in uncertainty in the
CSM.

Natural variation uncertainties:

Ecosystems include highly variable abiotic (e.g., weather, soils) and biotic
(e.g., population density) components. Only a fraction of the instances
can be sampled and known, thus leaving an uncertainty concerning the true
distribution of values.

Modeling uncertainty:

There is uncertainty over how well a model approximates true relationships
among site-specific environmental conditions and plants and animals. Models
tend to be relatively simple and normally only partially validated with
field tests.